Addresses specified in Internet Protocol version 4 (IPv4) (IPv4 addresses) have conventionally been used in communication via the Internet, and depletion of newly allocatable IPv4 addresses has recently become a problem. Accordingly, introduction of Internet Protocol version 6 (IPv6) which is allowed to allocate a greater number of addresses has been started, and various software (for example, application programs) and hardware (communication terminals such as personal computer and network relay devices such as router device) compatible with IPv6 have been proposed. The communication infrastructure includes a large number of devices which are not compatible with IPv6, so that communication terminals compatible with both IPv4 and IPv6, i.e., communication terminals having the IPv4/IPv6 dual stack structure, have been proposed as the device compatible with IPv6 (see, JP 2011-523793A and JP 2013-115639A).
Network relay devices that are connected with the communication terminal having the IPv4/IPv6 dual stack structure include a network relay device configured to selectively operate in one of an operation mode of operating as a router device and an operation mode of operating as a bridge device. This network relay device maintains the same operation mode irrespective of the version of an IP which an IP packet to be relayed is compliant with, so that a problem that one of an IPv4 packet and an IPv6 packet is not normally relayed may cause in some connection mode of the communication terminal via this network relay device (network configuration). Relay of a packet according to the version of the IP has, however, not been fully discussed so far.